For high-speed logic circuits in digital computers, applications of microstrips and striplines structures are very often and extensively used to the interconnection between these circuits. Moreover, for the high-speed data transmission rate, the differential mode of transmission lines with shielding planes is a necessary and important method for low voltage (3.3 Voltage) to reduce transmission errors induced by the noise during the data transmission. There is a requirement of one hundred ohm impedance for differential mode of transmission lines. Recently, automatic techniques and equipment are capable of fabricating these structures with the controllable impedance and transmitted timing of signal paths. However, the asymmetric structure of microstrips has a serious disadvantage because of the weakness of the mechanical properties for flexible cables, and it also permits significant levels of extraneous electromagnetic radiation.
For the symmetric structure of striplines, not only can it enhance the mechanical features of flexibility and anti-fatigue for flat flexible cables, but also greatly reduce effects of electromagnetic radiation. However, due to the symmetric structure of striplines, the additional ground reference plane greatly reduces the impedance of signal path because of the increasing capacitance between the signal path and the additional ground reference plane. To consider the high flexibility and anti-fatigue with repeated motion for the stripline type of flat flexible cables, the distance between the differential mode of signal paths and the ground reference plane should be designed to be shorter than that between the pair of the differential mode of signal paths. It also implies that the same design method is needed for avoiding increasing the thickness of the printed circuit board to maintain the desired impedance at the same time.
Ground planes or other voltage reference planes are positioned in planes parallel to the conductor planes in which the conductors are designed in plane in a flexible cable or a printed circuit board. It aims to control the impedance of the conductors and to block the transmission of electromagnetic radiation from conductors carrying high frequency signals. A solid ground plane is generally used in printed circuit boards or flexible cables, but it is inflexible except for the thin film type.
Another disadvantage for solid ground plane is that the impedance of signal lines may be lower than desired because the large capacitance is built up in the small spacing between the signal lines and a solid ground plane or the reference plane. The more we increase the spacing between the signal lines and a solid ground plane or the reference plane to reduce the capacitance and thereby increase the impedance of the signal lines, the thicker and thus less flexible and more likely to break with repeated motion. Similarly, a printed circuit board becomes thicker and thus more heavy and more costly to build.
Reference planes having conductive elements formed in a grid have been utilized in microstrip designs to increase the impedance and to provide flexibility. However, because the grid is not continuous like a solid reference plane, it has been found to be quite difficult to control both impedance and transmitted timing of signal lines protected by a grid reference plane with only one pattern.
One of the particular difficulties is to control the impedance of flexible cables and printed circuit boards utilizing grid reference planes, especially for stripline type cables with the structure of turns. Generally, when the orientation of a signal line needs to be changed by, for example, 90 degrees or the like, the turn is not incorporated into the signal line with a single 90-degree turn. Rather, the change in orientation is generally implemented with a curve such that the orientation of the signal line varies continuously from its original orientation to the new orientation. It is likely that the signal line will have different alignment with the conduction of the upper or the lower grid with both grids at various points in the turn. Such alignment causes a significant variation in impedance at such points and causes a substantial impedance discontinuity.
The impedance of microstrip and stripline construction is determined by the signal conductor width, the separation of the conductor from the reference planes, the dielectrics that surround the conductor and, to a lesser degree, the thickness of the conductor. However, such traditional methods of determining impedance in striplines and microstrips impose too many constraints on the designer. For example, the impedance needs to be twice of 100 ohms for the differential mode transmission lines. One way of obtaining such high impedance using existing technology is to increase the separation between the signal conductor and the reference plane. However, this would require the use of a thicker flexible cable with the less flexibility and anti-fatigue, different dielectric constant materials surrounding the conductor, or expensive printed circuit boards of nonstandard thickness. Such nonstandard printed circuit boards are not only expensive to implement, but also undesirable in many applications due to their thickness.
There is a disadvantage associated with traditional microstrip construction with high-speed transmission rate in that it generates both forward and reverse crosstalk, which can seriously deteriorate signal quality. Crosstalk is the effect of coupling the signal of one channel into another. Crosstalk may arise from a number of sources, one of which is the unbalance of cable parameters, in particular, the capacitance and inductance between conductors. Therefore, this inbalance may result in serious coupling from the signal of one conductor into another, and such unbalance is generally aggravated when a conductor is exposed to nonhomogeneous media, as is the case with traditional microstrip construction.
Solid surface conductors in traditional microstrip construction are most likely to radiate high levels of electromagnetic radiation to interfere with the functioning of surrounding electronics. Conversely, extraneous radiation may also affect the operation of surface conductors. In traditional microstrip construction with high-speed transmission rate, the surface of the conductor is free to radiate into the cavity of the system enclosing the circuit board, so it is difficult to provide adequate shielding. It implies that the structure of stripline is needed to reduce the containment of such radiation. However, desired high impedance conductors of stripline construction is very difficult to implement without drastically increasing the separation between reference planes and conductors. Such an undesired increase in thickness would cause problems for the case of thin circuit boards needed in notebook computers or other standard circuit board needed to reduce the cost.
Flexible reference planes are needed for a stripline type flexible cable to have the capability of thousands of flexures, to achieve desired impedance and transmitted timing that permit transfer of the signals without degrading the signal quality, and to provide an acceptable shielding capability. Due to the slow wave effect, the transmitted time on the solid plane is faster than that of the shielding with void opening patterns. When the difference among the length of the transmission line is large enough to have timing effect for high-speed transmission rate, the traditional method is to add extra equivalent length for the short transmission lines at some place to compensate this effect. But this traditional extra length compensation may induce an undesired electromagnetic radiation due to the discontinuity of the impedance.
Even though the transmission lines are the same lengths, sometimes, it is necessary to control the transmitted timing to avoid the significant harmonic mode of the high-speed transmission rate. The transmitted time is related to the dielectric constant of the surrounding material, the slow wave effect due to the shielding pattern, and the compatible long length of transmission line. If it equals the significant harmonic mode of the high speed of transmission rate, the harmonic mode of this signal will bounce back and forth between two ends of the transmission line to deteriorate the signal transmission.